Depending on the physical-chemical characteristics of the petroleum oil being exported in subsea lines, there occurs the well-known paraffination phenomenon, with the consequent loss of charge and increasing costs. The paraffin deposition phenomenon basically means the gradual and progressive deposition of paraffin from organic compounds throughout the lines. Thus, produced oils having paraffin contents higher than 3 weight % show high paraffin deposition potential when submitted to the flow conditions and to the characteristics of the subsea flowscheme.
The deposition process is closely related to the low temperatures of the sea bottom, to the temperature gradient set between the oil and the seawater throughout the subsea line, to the flowrates of oil and gas, to the shearing dispersion and to a lesser extent to the gravitational segregation and to the brownian movement. The generation of paraffin crystals is associated to the WAT (Wax Appearance Temperature) defined as ". . . the highest temperature at which is initiated the generation of a solid phase in oil, whenever oil is submitted to a previous thermal treatment and to a controlled cooling rate under isobaric conditions . . . "
U.S. Pat. No. 4,755,230 teaches applying the reaction of heat and nitrogen generation to the dewaxing of hydrocarbon transmission lines with the aid of an oil-in-water emulsion and HCI as an activator of the nitrogen and heat generation reaction. The process requires further an organic solvent added of a crystalline modifier in order to keep as low as possible the cloud point of the parfins in order to avoid their re-precipitation in the line after cooling. The use of the process requires that the paraffin-containing portion of the line be isolated from the rest of the line. The way the process is effected can yield a situation where the paraffinic deposit is simply displaced within the line, without being effectively withdrawn. Also, the direction of the pumping of the treating solution can cause a plug to be formed at one end of the line, so that failure of the line can occur due to excessive pressure.
G-B-B2276218 of the Applicant and herein fully incorporated as reference relates to the dewaxing of lines of up to 4 inches in diameter and 10 km in length, with the aid of a Nitrogen Generating System/Emulsion using acetic acid as a delayed-action activator. The solutions of nitrogen and heat-generating salts are prepared in a process ship, each salt solution being prepared in a separate vessel.
U.S. patent application Ser. No. 08/742,126 of Oct. 31, 1996 of the Applicant and herein fully incorporated as reference relates to the dewaxing of lines of up to 50,000 meters length and internal diameter of up to 12 inches using a Nitrogen Generating System where the delayed-action activator of the reaction of nitrogen and heat generation is a polyadipic anhydride of controlled hydrodegradability. In the process taught in U.S. Ser. No. 08/742,126 the solutions of nitrogen salts are prepared in one single vessel, on the production platform. The salt-containing solution is stabilized by the addition of NaOH and emulsified with an organic solvent. The delayed-action activator is added on flow to the nitrogen salts emulsion. The organic solvent is, for example, kerosene. The requirements of the process comprise emptying or otherwise displacing the whole amount of oil contained in the line. The treating fluid is then pumped into the line, either from a rig, the platform itself or through an auxiliary line; the fluid is allowed to act on the paraffin deposit for the period of time which is required for the development of the chemical reaction between the nitrogen salts and for the fluidization of the deposit. The spent fluids and the emulsified paraffin are recovered and well production is resumed.
Therefore, the state-of-the-art processes either require expensive ship services as in GB-B-2276218 or the interruption of the oil production as in U.S. Ser. No. 08/742,126, this representing serious drawbacks to the economics of the process. Further, those processes cannot be applied to oil export pipelines which are devoid of alternatives of discharge of produced oil during the dewaxing treatment.
Thus, in the art of dewaxing of lines there is the need of a thermal-chemical process using a Nitrogen Generating System for the dewaxing of large dimension oil export pipelines, where the solution of nitrogen and heat-generating salts be mixed with the as-produced oil. This would avoid interrupting the oil production as well as the need of expensive petroleum fractions as solvents, which represents great savings. Such a process is disclosed and claimed in the present invention.